- The polysaccharide produced by the red microalga Porphyridium sp. was highly inhibitory for cell transformation of mouse fibroblast cells by an MSV-124 virus stock. This inhibition was most effective if the polysaccharide was added 2 h before or at the time of infection. The finding that the inhibition of cell transformation by MuSV-124 was reversible after removal of the polysaccharide suggested that Porphyridium sp. polysaccharide inhibited a late step after provirus integration into the host genome. Addition of the polysaccharide post-infection significantly reduced the number of transformed cells, but its effect was less marked than that obtained when the polysaccharide was added before or at the time of infection. These findings support the possibility that the polysaccharide may affect early steps in virus replication cycle, such as virus absorption into the host cells, in addition to its effect on a late step after provirus integration.